The invention relates generally to exciter circuits for ignition systems used with internal combustion engines. More particularly, the invention relates to exciter circuits that utilize solid-state switches such as, for example, thyristors, as control devices for spark rate timing.
A conventional ignition system for an internal combustion engine, such as, for example, a gas turbine aircraft engine, includes a charging circuit, a storage capacitor, a discharge circuit and at least one igniter plug located in the combustion chamber. The discharge circuit includes a switching device connected in series between the capacitor and the plug. For many years, such ignition systems have used spark gaps as the switching device to isolate the storage capacitor from the plug. When the voltage on the capacitor reaches the spark gap breakover voltage, the capacitor discharges through the plug and a spark is produced.
More recently, turbine engine and aircraft manufacturers have become interested in replacing the spark gap with a solid-state switch, such as an SCR or thyristor. This is due, in part, because a solid state switch typically operates longer than a spark gap tube which may exhibit electrode erosion. Also, solid state switches are produced in large volume making them less expensive than spark gaps which are individually crafted in small quantities. Furthermore, the storage capacitor's voltage at discharge remains essentially constant over the life time of the solid state switch, but can change significantly during the life of the spark gap due to electrode erosion.
However, there are also significant disadvantages to replacing a spark gap with a solid state switch. One concerns the peak power produced by the spark discharge pulse. Although spark energy is about the same for the spark gap and solid state switch designs, peak spark power is severely reduced using known solid state switch designs because the solid state switch limits peak discharge current to about 1000 amps with a current transition rate (i.e. di/dt) limit of about 200 amps/.mu.second. In contrast, spark gap discharge currents rise rapidly at about 1000 amps/.mu.second to a peak of about 2000 amps. This produces a high peak power that causes a loud bang and sonic shock wave that emanates from the igniter tip. It is this shock wave that breaks up and disperses the fuel particles making them easier to ignite. The high peak current and current transition rates required for high peak power do not present a problem for spark gaps but are of a destructive nature for present solid state thyristors.
When a solid state switch such as, for example, a thyristor, is initially gated on, only a very small portion of the die area around the gate electrode attachment conducts current due to a finite spreading velocity. If a fast rising current is permitted at turn on, a high current density occurs in the small conducting area of the die resulting in high switching losses. These high losses create excessive heating and are of a destructive nature to the thyristor device. To allow proper current spreading of the entire die area which will permit a safe operating environment for the thyristor, a saturable core inductor, often referred to as a delay reactor, must be incorporated in the circuit design. The delay reactor is connected in series with the thyristor switch, and the inductance of the reactor limits the rate of rise of the current (di/dt) for a period of time while the thyristor is turning on. Once the thyristor is in full conduction, the delay reactor's core saturates and the inductance becomes so small that it no longer affects the circuit operation.
If too high a di/dt level is being applied to a conventional thyristor device, the thyristor will eventually and gradually become leaky, and a reduction in the breakover voltage will slowly occur. The rate at which these changes take place is dependent upon how high the di/dt levels are that the switching device experiences over time.
Based on testing that has been conducted by engine manufacturers on ignition systems that employ solid-state technology, ignition lightoff has been a problem and a concern. It is believed that these no lightoff conditions are caused by at least two characteristic differences. One is that the reduced peak power level is not sufficient to maintain a clear plug, thereby resulting in the absence of a spark due to contamination fouling. The second condition results in less of a shock wave being developed, as a result of the peak power reduction, which may not be sufficient for igniting the fuel particles under more severe fuel-air ratios and contaminated mixtures.
Another disadvantage to present solid state switch designs is that leakage current of conventional thyristors increases significantly at high operating temperatures. These leakage currents act as load on the charging circuitry and divert charging current away from the main storage capacitor. This causes the spark rate to decrease. To maintain a constant spark rate, known exciter designs must utilize additional timing and regulating circuitry to compensate for the leakage problem.
Thus, there is a present need for a simple and reliable exciter, preferably using solid-state switches, that produces high energy sparks with high peak power at a constant spark rate without switch degradation.